This invention relates to acoustic apparatus for use in the imaging or characterization of biological tissue and, more particularly, to an ultrasonic fan beam scanner for time-of-flight computerized tomography which is especially useful in breast and other soft tissue examination.
Computerized x-ray tomographic systems are at present well known and provide to medical practitioners high accuracy measurements of x-ray absorption coefficients in soft tissue. These scanners utilize ionizing radiation, however, and may be potentially destructive to the imaged tissue when used for human examination. General information on these techniques is given in a magazine article entitled "Image Reconstruction from Projections" in Scientific American, Oct. 1975, Vol. 233, No. 4, pp. 56-68.
Ultrasonic imaging systems are noninvasive and are nondestructive so far as is presently known. The most widely used technique is the pulse echo B-scan mode, or variants thereof, in which a video display employs intensity modulation to depict echo return amplitude as a function of depth. The echoes are generated at interfaces between media of different acoustic impedance within the specimen tissue. However, the magnitude of the interfacial reflection coefficient depends not only on the impedance ratio, but also on the angle of incidence of the beam, and furthermore the echo amplitude is reduced due to attenuation from the intervening tissue. Thus, the images generated by B-scan systems describe geometry and topology of interfaces, which can be interpreted qualitatively but do not give quantitative information on bulk tissue due to geometric distortion of the echo amplitude arising from refraction, specular reflection, and diffraction. Similarly, ultrasound transmission systems based on amplitude suffer from the same limitations. As applied to human breast imaging by echography, it has been observed that characteristic echo patterns accompany various types of breast leasions, and clinical diagnosis is thus effected mainly by a subjective pattern recognition process.
The problems associated with absorption or reflection amplitude are eliminated by a system based on propagation velocity of the ultrasonic wave, which is easily measured by detecting the time of passage of an ultrasound pulse transmitted through the specimen over a known path length. The concept of computerized time-of-flight (TOF) tomography for reconstruction of two-dimensional velocity distributions using a pencil beam geometry is discussed in the article by J. F. Greenleaf et al, entitled "Albebraic Reconstruction of Spatial Distributions of Acoustic Velocities in Tissue From Their Time of Flight Profiles," Acoustical Holography, Plenum Press, Vol. 6, pp. 71-89 (1975). This apparatus employed diametrically opposing transducers which were rectilinearly scanned while rotating the transducers incrementally between scans. Such an instrument is not clinically acceptable, however, because of the relatively long scan times and attendant poor image quality caused by patient motion during the scan.